A tandem color printer having a detachably mountable tandem drum unit is well known. Four photosensitive drums corresponding to four colors (yellow, magenta, cyan and black) are supported to the drum unit. The photosensitive drums are thus integrally detachably mountable in a main casing of the color printer.
The drum unit includes four drum sub-units, a front beam, a rear beam and a pair of side frames. Each drum sub-unit has a sub-unit frame that supports the photosensitive drum. The four drum sub-units are juxtaposed between the front beam and the rear beam in a front-to-rear direction. The four drum sub-units, the front beam and the rear beam are integrally interposed between the pair of side walls in a left-to-right direction.
Each drum sub-unit is formed with a space at a front side thereof. Each space is used as a developing cartridge accommodating section for accommodating therein a developing cartridge including a developing roller. The developing roller has a developing roller shaft that is rotatably supported to a casing of the developing cartridge. The developing roller shaft has widthwise ends protruding outward from the casing of the developing cartridge. Each of the pair of side walls of the drum unit is formed with four guide grooves at positions in confrontation with the four developing cartridge accommodating sections for guiding loading of the developing cartridge into the developing cartridge accommodating section.
Upon loading of the developing cartridge into the developing cartridge accommodating section, the widthwise ends of the developing roller shaft are inserted into the corresponding guide grooves from above. The developing cartridge is then slidingly moved toward the developing cartridge accommodating section while being guided along the guide grooves. When the developing roller is brought into contact with the corresponding photosensitive drum, the developing cartridge is prevented from moving further. The developing cartridge is then tilted (pivotally moved) about the developing roller shaft toward the neighboring drum sub-unit positioned frontward or toward the front beam.
The front beam and the sub-unit frame of each drum sub-unit respectively have surfaces against which the developing cartridge is tilted. On these surfaces of the front beam and the sub-unit frame, two supporting rollers are disposed in separation from each other in an axial direction of the photosensitive drum. When the developing cartridge leans on the front beam or on the drum sub-unit, the casing of the developing cartridge is brought into contact with the supporting rollers. The developing cartridge is thus supported within the developing cartridge accommodating section at four contact points: at two contact points where the widthwise ends of the developing roller shaft are supported by the pair of side frames of the drum unit; and at two contact points where the casing of the developing cartridge is supported by the two supporting rollers.